This invention relates to a method of, and apparatus for, grooving a flat thin material, comprising fibres aligned in at least one direction, such as cardboard, paper and all composite materials of this kind.
Hitherto, the grooving or scoring of cardboard to enable it to be subsequently folded has been carried out on large platen presses operating on the flat material, in the case of small-scale production, or on rotary machines in the case of production on a larger scale. In both of these cases, the problem of grooving in the direction of the fibres of the cardboard has arisen.
The purpose of grooving is to separate the lines of fibres so that upon subsequent folding of the cardboard along the fold-line or fold-lines provided, these fibres do not form a compact body that offers excessively great resistance to folding. Cardboard has a fibrous structure consisting of a large number of longitudinal fibres disposed very close to each other, and a considerably smaller number of transverse fibres which are spaced relatively widely from each other.
Grooving of the cardboard does not raise any difficulties in the direction transverse to that of the fibres, but this is not so in the longitudinal direction. For the purpose of grooving, use is generally made of a tool in the form of a blade which has a rounded, rectangular pointed or trapezoidal end portion and which is displaced relatively to a hollow zone formed in a platen acting as a backing member. The end of the blade terminates substantially in the plane of the oppositely disposed face of the backing member, and in this zone the lateral surface of the blade is located at a distance from edges which delimit the hollow zone in the backing member, which distance is substantially equal to the thickness of the cardboard sheet. Thus, the blade pushes that portion of the cardboard that faces it into the hollow portion in the backing member, and said portion of the cardboard is then firmly applied against the lateral surface of the blade. This grooving operation results in division of the lines of fibres i.e. in separation of the fibres, and in the scoring of a concave folding line in the cardboard. Upon subsequent folding of the cardboard, the outer edge of the fold is formed by a tensioned portion which should not break open for reasons of appearance as well as of strength.
Though the separation of the lines of fibres in the transverse direction does not pose any particular problem, this is not the case in the longitudinal direction. In the latter case, separation of the lines of fibres occurs in a considerably less effective manner, and the force required for subsequently folding the cardboard is relatively great.
It is generally considered that the force necessary for longitudinally folding cardboard that is grooved transversely of the fibres should be less than 50 percent of the force necessary for folding ungrooved cardboard, whereas the force required for transversely folding cardboard that has been longitudinally grooved in relation to the fibres should be less than 60 percent of the force necessary for folding ungrooved cardboard. Generally however, in the case of poor-quality cardboard, which is preferred for industrial purposes since it is the cheapest, it will be found that the force necessary for folding the cardboard in the transverse direction, that is to say parallel to the longitudinal fibres, may be as much as ninety percent of the force necessary for folding the ungrooved cardboard, because of inefficient division or separation of the fibres in this direction.
For the purpose of achieving efficient division in the direction of the longitudinal fibres, it has been proposed to use a pointed blade which partly penetrates into the cardboard and nicks the fibres contained in the cardboard To prevent tearing of the cardboard, the edge of the pointed blade is interrupted, by means of a milling operation, at regular intervals so as to form, in the cardboard, dotted scorelines with approximately 2 millimeters separating each pair of adjacent elongate dots. These dotted lines are formed in the inner portion of the groove so that they appear on the outer surface of the fold when this is formed in the cardboard. These dotted lines result in hollow portions which impart an unattractive appearance to the edge of the fold and which tend to open up so that a zone of weakness is created. Although this method can be used quite readily on a flat press, it may be difficult to apply in a rotary grooving machine.
The present invention aims at overcoming these various difficulties by providing a grooving method and apparatus which are simple to use and which enable a finished product of better quality to be obtained.